All phenylpropanoids are derived from cinnamic acid, which is formed from phenylalanine by the action of phenylalanine ammonia-lyase, the branch point between primary and secondary metabolism. Isoflavones represent a class of secondary metabolites produced in plants by the phenylpropanoid biosynthesis pathway. The biosynthetic pathway for free isoflavones and their relationship with several other classes of phenylpropanoids is presented in FIG. 1.
Chalcone synthase (E.C. 2.3.1.74) is the first enzyme in the flavonoid-specific branch of the phenylpropanoid biosynthetic pathway (Dixon et al., Plant Cell 7:1085–1097 (1995)). As such it is a key enzyme in secondary metabolism. Chalcone synthase catalyzes the condensation of p-coumaroyl-coenzyme A and three molecules of malonyl-coenzyme A. In most plant families, the initial product of chalcone synthase is a tetrahydroxychalcone, which is further converted into other flavonoid classes. In legumes, which possess chalcone reductase as well as chalcone synthase, a trihydroxychalcone may be formed. Compounds resulting from chalcone synthase are important in several physiological processes, including wound response, defense against microbial infection via production of isoflavonoid phytoalexins, UV-B protectants and nodulation in legumes.
Chalcone synthase (Registry Number 180391-56-4) has been associated with expression in anthers of rice (Oryza sativa) at the uninucleate microspore stage (Hihara et al., Plant Mol. Biol. 30(6):1181–1193 (1996)). Male sterility may be induced by the inhibition of flavenoid biosythesis using an expression vector that produces antisense RNA for the 3′ untranslated region of chalcone synthase A gene (see U.S. Pat. No. 6,037,523, column 12, lines 21–25, referencing Van der Meer, et al., The Plant Cell, Vol. 4; pp. 253; (1992)). In soybean (Glycine max), six isoforms of chalcone synthase originating from different messenger RNAs have been reported (Wingender et al., Mol. Gen. Genet. 218:315–322 (1989)). To date, no chalcone synthase has been isolated from wheat (Triticum aestivum).